As a floor structure of a vehicle body, a structure (Japanese Examined Utility Model Registration Application Publication No. Hei 3-8542, for example) has been known in which a longitudinally extending floor tunnel is provided. The floor tunnel not only forms space for arranging a configuration extending in the longitudinal direction of the vehicle body such as a fuel pipe and an exhaust pipe, but also improves stiffness of the floor.
Due to its sectional shape, a floor tunnel may not be sufficiently stiff to withstand load in the direction of opening and closing its right and left side walls to the right and left. Hence, sometimes vertical vibration of right and left floors of the floor tunnel cannot be suppressed sufficiently, or there may be drawbacks in load distribution at the time of a side crash, for example. Although a brace connecting bottom parts of right and left floor panels may be provided to pass through a lower part of the floor tunnel as a countermeasure, this causes increase in the number of parts and in the weight of the vehicle body.